A doctor who was under pressure because his department was short-staffed accidentally administered a fatal dose of calcium to a baby girl, an inquest heard yesterday.

Abbie Hattam, who was five months old, died in her mother's arms after a heart operation went wrong. Tests found she had been given 10 times the amount of calcium she was supposed to receive as part of the procedure. The girl's mother, Dawn Hattam, 32, of Truro in Cornwall, told the inquest in Bristol that her daughter, a twin, was a "little fighter" who had her chance at life cruelly taken away.

Ms Hattam said: "I had absolute faith in the doctors. During the surgery I just couldn't wait to see her open her eyes again and look at me. I understand someone made a mistake. I was so mad and angry and shocked that Abbie was gone."

Abbie and her sister, Teagan, were born three and a half months premature. Abbie, who weighed only 1lb 15oz, had a hole in her heart and suffered from high blood pressure in the arteries supplying her lungs. She went to Bristol Children's hospital in May 2005 for a heart operation.

The procedure, described at the inquest as "technically simple" was cancelled twice. It finally went ahead on May 25. Staff told Ms Hattam and her husband, Jason, that the 90-minute operation had gone well and they felt happy enough to go out for a meal. But an hour after the operation the hospital called them and told them their daughter was unwell. The consultant in charge of the paediatric ward, Dr Patricia Weir, said that initially it was not known what had gone wrong. Dr Weir said: "I looked at the paperwork from the operation and found that calcium levels were unexpectedly high during and post operation. Abnormally high calcium levels caused constriction of the blood vessels which would lead to brain damage."

She said the high amount of calcium in the primer - a blood mix administered during the operation - had probably killed Abbie, who died the day after the operation. The primer was said to have been made up by Richard Downes, who joined the hospital in 1995 and was accredited with making the cardiac unit one of the best in the country.

Ashwinikumar Pawade, who carried out the operation, said Mr Downes was under pressure because his department, the perfusion department, had to cover the children's hospital and the Bristol Royal Infirmary. Mr Pawade added: "The perfusion department has been short-staffed for a long time and has struggled to keep up with an increasing workload."

He said he had argued with Mr Downes on the day of the operation because Mr Downes had wanted it cancelled as it was late in the afternoon. The operation was scheduled to start at 5.15pm, but Mr Downes said he intended to leave the hospital by 5.30pm, Mr Pawade claimed.

Mr Downes was arrested by police, but released without charge. Detective Constable Rob Carter, of Avon and Somerset police, told the inquest he believed a calcium overdose was responsible for Abbie's death. The hearing continues.